Magnetoresistive element and magnetic memory

ABSTRACT

A magnetoresistive element according to an embodiment includes: a base layer; a first magnetic layer formed on the base layer, and including a first magnetic film having an axis of easy magnetization in a direction perpendicular to a film plane, the first magnetic film including Mn x Ga 100-x  (45≦x&lt;64 atomic %); a first nonmagnetic layer formed on the first magnetic layer; and a second magnetic layer formed on the first nonmagnetic layer, and including a second magnetic film having an axis of easy magnetization in a direction perpendicular to a film plane, the second magnetic film including Mn y Ga 100-y  (45≦y&lt;64 atomic %). The first and second magnetic layers include different Mn composition rates from each other, a magnetization direction of the first magnetic layer is changeable by a current flowing between the first magnetic layer and the second magnetic layer via the first nonmagnetic layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No. 13/310,154filed Dec. 2, 2011, and is based upon and claims the benefit of priorityfrom prior Japanese Patent Application No. 2011-68868 filed on Mar. 25,2011 in Japan, the entire contents of each of which are incorporatedherein by reference.

FIELD

Embodiments described herein relate generally to a magnetoresistiveelement and magnetic memory.

BACKGROUND

It is known that a MTJ (Magnetic Tunnel Junction) element serving as amagnetoresistive element has a stacked structure as a basic structure,and shows a tunneling magnetoresistive (TMR) effect. The stackedstructure is formed by a first ferromagnetic layer, a tunnel barrierlayer, and a second ferromagnetic layer. Such MTJ elements are used in100 bits per square inch (Mbpsi) HDD heads and magnetic random accessmemories (MRAMs).

A MRAM characteristically stores information (“1”, “0”) depending onchanges in the relative angle of the magnetizations of magnetic layersincluded in each MTJ element, and therefore, is nonvolatile. Since amagnetization switching takes only several nanoseconds, high-speed datawriting and high-speed data reading can be performed. Therefore, MRAMsare expected to be the next-generation high-speed nonvolatile memories.If a technique called a spin torque transfer switching technique forcontrolling magnetization through spin-polarized current is used, thecurrent density can be made higher when the cell size of the MRAM ismade smaller. Accordingly, high-density, low-power-consumption MRAMsthat can readily invert the magnetization of a magnetic material can beformed.

Furthermore, in recent years, attention has been drawn to the theorythat a magnetoresistance ratio as high as 1000% can be achieved by usingMgO as the tunnel barrier layer. Since the MgO is crystallized,selective tunneling conduction of the electrons having a certainwavenumber from the ferromagnetic layer can be performed, and thoseelectrons can keep the wavenumber during that time. At this point, thespin polarization has a large value in a certain crystallineorientation, and therefore, a giant magnetoresistive effect appears.Accordingly, an increase in the magnetoresistive effect of each MTJelement leads directly to a higher-density MRAM that consumes lesspower.

Meanwhile, to form high-density nonvolatile memories, high integrationof magnetoresistive elements is essential. However, the ferromagneticbodies forming magnetoresistive elements have thermal disturbanceresistance that is degraded with a decrease in element size. Therefore,how to improve the magnetic anisotropy and thermal disturbanceresistance of each ferromagnetic material is a critical issue.

To counter this problem, trial MRAMs using perpendicular-magnetizationMTJ elements in which ferromagnetic bodies have magnetization directionsperpendicular to the film plane have been made in recent years. In aperpendicular-magnetization MTJ element, a material having high magneticcrystalline anisotropy is normally used for ferromagnetic bodies. Such amaterial has a magnetization in a certain crystal direction, and themagnetic crystalline anisotropy of the material can be controlled bychanging the composition ratio between constituent elements and thecrystallinity of the constituent elements. Accordingly, themagnetization direction can be controlled by changing the direction ofcrystal growth. Also, since ferromagnetic bodies have high magneticcrystalline anisotropy, the aspect ratio of the element can be adjusted.Furthermore, having high thermal disturbance resistance, ferromagneticbodies are suited for integration. In view of those facts, to realize ahighly-integrated MRAM that consumes less power, it is critical tomanufacture perpendicular-magnetization MTJ elements that have a greatmagnetoresistive effect.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a magnetoresistive element accordingto a first embodiment;

FIG. 2 is a diagram showing a variation in the coercive force in aperpendicular direction with respect to the Mn composition rate in aMnGa layer;

FIG. 3 is a cross-sectional view of a magnetoresistive element accordingto a second embodiment;

FIG. 4 is a cross-sectional view of a magnetoresistive element accordingto a third embodiment;

FIG. 5 is a cross-sectional view of a magnetoresistive element accordingto a fourth embodiment;

FIG. 6 is a cross-sectional view of a magnetoresistive element accordingto a fifth embodiment;

FIG. 7 is a cross-sectional view of a specific example of the baselayer;

FIG. 8 is a diagram showing an example of the magnetization curves ofMn₇₅Ga₂₅;

FIG. 9 is a diagram showing another example of the magnetization curvesof Mn₇₅Ga₂₅;

FIG. 10 is a circuit diagram of a MRAM according to a sixth embodiment;and

FIG. 11 is a cross-sectional view of a memory cell of the MRAM accordingto the sixth embodiment.

DETAILED DESCRIPTION

A magnetoresistive element according to an embodiment includes: a baselayer; a first magnetic layer formed on the base layer, and including afirst magnetic film having an axis of easy magnetization in a directionperpendicular to a film plane, the first magnetic film includingMn_(x)Ga_(100-x) (45≦x<64 atomic %); a first nonmagnetic layer formed onthe first magnetic layer; and a second magnetic layer formed on thefirst nonmagnetic layer, and including a second magnetic film having anaxis of easy magnetization in a direction perpendicular to a film plane,the second magnetic film including Mn_(y)Ga_(100-y) (45≦y<64 atomic %).The first and second magnetic layers include different Mn compositionrates from each other, a magnetization direction of the first magneticlayer is changeable by a current flowing between the first magneticlayer and the second magnetic layer via the first nonmagnetic layer.

The following is a description of embodiments, with reference to theaccompanying drawings. In the following description, components havingthe same functions and structures are denoted by the same referencenumerals, and explanation of them will be repeated only where necessary.

First Embodiment

FIG. 1 shows a magnetoresistive element according to a first embodiment.FIG. 1 is a cross-sectional view of the magnetoresistive element 1 ofthe first embodiment. The magnetoresistive element 1 of this embodimentis a MTJ element, and has a structure that is formed by stacking aferromagnetic layer 2, a nonmagnetic layer 4 (hereinafter also referredto as the tunnel barrier layer 4), an interfacial magnetic layer 6, anda ferromagnetic layer 8 in this order on a base layer 100. The baselayer 100 is used to control the crystalline properties such as thecrystalline orientations and grain sizes of the ferromagnetic layer 2and the layers located above the ferromagnetic layer 2, and thecharacteristics of the base layer 100 will be described later in detail.The ferromagnetic layer 2 contains Mn and Ga, and the composition ratiobetween the Mn and Ga is Mn_(x)Ga_(100-x) (45≦x<64 atomic %). FIG. 2shows the variation in the coercive force in a vertical direction withrespect to the Mn composition rate in a structure expressed as “5-nmthick Ta/30-nm thick MnGa/Cr/MgO substrate”. It should be noted that theleft-hand side of each symbol “/” indicates an upper layer, and theright-hand side of each “/” indicates a lower layer. When thecomposition ratio is Mn₅₀Ga₅₀, the coercive force is approximately 1kOe. As the Mn composition rate is increased, the coercive force becomesgreater, and increases to approximately 6 kOe when the composition ratiois Mn₆₀Ga₄₀. The samples used here were formed by performingco-sputtering on a sample of Mn₄₅Ga₅₅ and a Mn target, andsystematically varying the composition rates. Here, the compositionrates of Mn and Ga are designed values. In a case where MnGa is formedby a sputtering technique, MnGa targets are normally used. However, itis very difficult to form MnGa targets that cover all compositionratios. When the Mn composition rate is decreased, the target cracks.The limit composition rate of Mn to Ga at which a target can be formedwithout a crack in the MnGa is approximately 45 atomic %. Therefore,Mn₄₅Ga₅₅ is set as the lower limit of the Mn composition rate. Theresistance value of a MTJ element which comprises two magnetic layersand a tunnel barrier interposed between the two magnetic layers isdetermined by the angle between the magnetization directions of the twomagnetic layers. The angle between the magnetization directions can becontrolled by adjusting an external magnetic field or the current to beapplied to the element. In that case, the difference in coercive forcebetween the two magnetic layers is made larger, so that the anglebetween the magnetization directions can be more stably controlled.Here, the magnetic layer with the greater coercive force is referred toas the reference layer, and the magnetic layer with the smaller coerciveforce is referred to as the recording layer. Therefore, a Magnetic layerto be used as the reference layer is normally expected to have a greatcoercive force, and a magnetic layer to be used as the recording layeris normally expected to have a small coercive force. Since the coerciveforce can be increased by increasing the Mn concentration as shown inFIG. 2, such a layer is more suited to be the reference layer.

A MnGa phase diagram shown in the reference (Thaddeus B. Massalski,“BINARY ALLOY PHASE DIAGRAMS”, Vol. 2, p. 1144) suggests that differentphases coexist in the composition domain where the Mn concentration isapproximately 65% or higher. In view of that, crystal growth ispredicted in a situation where phases with different magnetic propertiescoexist in the composition domain, and this might result in variationsof magnetic properties. Therefore, a MnGa magnetic material was stablyformed, and, to achieve desired magnetic properties, the upper limit ofthe composition rate of Mn to Ga was set at 64 atomic %. Meanwhile, theferromagnetic layer 8 is a ferromagnetic layer that does not contain Mnand Ga, and the characteristics of the ferromagnetic layer 8 will bedescribed later in detail. In the first embodiment, MgAlO is preferablyused as the nonmagnetic layer 4. For example, in a case where a stackedstructure is formed by stacking a ferromagnetic layer made of MnGa, anonmagnetic layer made of crystalline MgO, and a ferromagnetic layermade of MnGa in this order, the epitaxial relationship, MnGa (001)/MgO(001)/MnGa (001), can be formed. Here, MnGa (001) and MgO (001) meanthat crystals are oriented so that the (001) surfaces are exposed in thenormal directions of the respective films. Accordingly, the wavenumberselectivity of tunneling electrons can be improved, and a highmagnetoresistance ratio can be achieved. In a case where MnGa hascrystalline orientation so that the (001) surface is exposed in thenormal direction of the film, the lattice constant in the film in-planedirection is approximately 3.91 Å, regardless of the composition ratiobetween Mn and Ga. The lattice mismatch determined from the latticeconstants of bulks in the film in-plane directions of MnGa and MgO is aslarge as approximately 7.7%. The lattice mismatch is defined by thefollowing mathematical formula: (a(MgO)−a(MnGa))/a(MnGa)×100. Here,a(MgO) and a(MnGa) represent the lattice constants of MnGa and MgO inthe film in-plane directions, respectively. If the lattice mismatch islarge, a dislocation or the like is caused in an interface so as toreduce the interfacial energy generated due to lattice distortion. Inthat case, an epitaxial relationship is formed between crystal grains,and it is difficult to cause uniform epitaxial growth in the film plane.When a current is applied to the element, a dislocation becomes thescattering source of electrons, and the magnetoresistance ratio becomeslower. Therefore, to cause uniform epitaxial growth in the film planewithout a dislocation, it is essential to form a stacked structure withmaterials having a small lattice mismatch. Therefore, MgAlO is used asthe nonmagnetic layer 4, and MnGa (001)/MgAlO (001)/MnGa (001) isformed. For example, the lattice constant of MgAl₂O₄ in the a-axisdirection is 8.09 Å. Accordingly, the lattice mismatch can be reduced to3.5%, and a higher magnetoresistance ratio is achieved.

The ferromagnetic layer 2 and the ferromagnetic layer 8 each have aneasy magnetization direction perpendicular to the film plane. That is,the MTJ element 1 of this embodiment is a so-calledperpendicular-magnetization MTJ element in which the ferromagnetic layer2 and the ferromagnetic layer 8 each have a magnetization directionperpendicular to the film plane. It should be noted that, in thisspecification, “film plane” refers to the upper surface of eachferromagnetic layer. Also, “easy magnetization direction” is thedirection in which a macro-size ferromagnetic material has the lowestinternal energy when the spontaneous magnetization direction of themacro-size ferromagnetic material is the same as the direction in asituation where there are no external magnetic fields. On the otherhand, “hard magnetization direction” is the direction in which amacro-size ferromagnetic material has the largest internal energy whenthe spontaneous magnetization direction of the macro-size ferromagneticmaterial is the same as the direction in a situation where there are noexternal magnetic fields.

When a write current is applied to the MTJ element 1, the magnetizationdirection of one of the ferromagnetic layer 2 and the ferromagneticlayer 8 remains the same i.e. fixed before and after writing, and themagnetization direction of the other one is changeable. Theferromagnetic layer having a fixed magnetization direction is alsoreferred to as the reference layer, and the ferromagnetic layer having achangeable magnetization direction is also referred to as the recordinglayer. In this embodiment, the ferromagnetic layer 2 is the recordinglayer, and the ferromagnetic layer 8 is the reference layer, forexample. A write current flowing in a direction perpendicular to thefilm plane is applied between the ferromagnetic layer 2 and theferromagnetic layer 8. In a case where the ferromagnetic layer 2 is therecording layer, the ferromagnetic layer 8 is the reference layer, andthe magnetization direction of the ferromagnetic layer 2 is antiparallelto (or the opposite of) the magnetization direction of the ferromagneticlayer 8, a write current is applied from the ferromagnetic layer 2toward the ferromagnetic layer 8. In that case, electrons flow from theferromagnetic layer 8 to the ferromagnetic layer 2 via the interfacialmagnetic layer 6 and the nonmagnetic layer 4. The electrons that arespin-polarized through the ferromagnetic layer 8 flow into theferromagnetic layer 2. The spin-polarized electrons having spins in thesame direction as the magnetization direction of the ferromagnetic layer2 pass through the ferromagnetic layer 2. However, the spin-polarizedelectrons having spins in the opposite direction from the magnetizationdirection of the ferromagnetic layer 2 apply a spin torque to themagnetization of the ferromagnetic layer 2, to change the magnetizationdirection of the ferromagnetic layer 2 to the same direction as themagnetization direction of the ferromagnetic layer 8. Accordingly, themagnetization direction of the ferromagnetic layer 2 is switched, andbecomes parallel to (or the same as) the magnetization direction of theferromagnetic layer 8.

In a case where the magnetization direction of the ferromagnetic layer 2and the magnetization direction of the ferromagnetic layer 8 areparallel, on the other hand, a write current is applied from theferromagnetic layer 8 to the ferromagnetic layer 2. In that case,electrons flow from the ferromagnetic layer 2 to the ferromagnetic layer8 via the nonmagnetic layer 4 and the interfacial magnetic layer 6. As aresult, the electrons that are spin-polarized through the ferromagneticlayer 2 flow into the ferromagnetic layer 8. The spin-polarizedelectrons having spins in the same direction as the magnetizationdirection of the ferromagnetic layer 8 pass through the ferromagneticlayer 8. However, the spin-polarized electrons having spins in theopposite direction from the magnetization direction of the ferromagneticlayer 8 are reflected by the nonmagnetic layer 4, and flow into theferromagnetic layer 2 through the interfacial magnetic layer 6 and thenonmagnetic layer 4. As a result of this, a spin torque is applied tothe magnetization of the ferromagnetic layer 2, to change themagnetization direction of the ferromagnetic layer 2 to the oppositedirection from the magnetization direction of the ferromagnetic layer 8.Accordingly, the magnetization direction of the ferromagnetic layer 2 isswitched, and becomes antiparallel to the magnetization direction of theferromagnetic layer 8. It should be noted that the interfacial magneticlayer 6 is employed to increase spin polarization.

In the first embodiment, the ferromagnetic layer 2, the nonmagneticlayer 4 (hereinafter also referred to as the tunnel barrier layer 4),the interfacial magnetic layer 6, and the ferromagnetic layer 8 arestacked in this order on the base layer 100. However, those layers canbe stacked in reverse order on the base layer 100. That is, theferromagnetic layer 8, the interfacial magnetic layer 6, the nonmagneticlayer 4, and the ferromagnetic layer 2 can be stacked in this order onthe base layer 100.

Second Embodiment

FIG. 3 shows a magnetoresistive element according to a secondembodiment. FIG. 3 is a cross-sectional view of the magnetoresistiveelement 1A of the second embodiment. The magnetoresistive element 1A ofthis embodiment is a MTJ element, and has a structure that is formed bystacking a ferromagnetic layer 2, an interfacial magnetic layer 3, anonmagnetic layer 4, an interfacial magnetic layer 6, and aferromagnetic layer 8 in this order on a base layer 100. Theferromagnetic layer 2 contains Mn and Ga, and the composition ratiobetween the Mn and Ga is Mn_(x)Ga_(100-x) (45≦x<64 atomic %). MnAl ispreferably used as the interfacial magnetic layer 3, so as to increasespin polarization. As the nonmagnetic layer 4, the later describedtunnel barrier layer material can be used, but it is preferable to useMgAlO. As the interfacial magnetic layer 3, MnGa can also be used.

As in the first embodiment, each of the ferromagnetic layer 2 and theferromagnetic layer 8 has magnetic anisotropy in a directionperpendicular to the film plane, and has an easy magnetization directionperpendicular to the film plane. That is, the MTJ element 1A of thisembodiment is a so-called perpendicular-magnetization MTJ element inwhich the ferromagnetic layer 2 and the ferromagnetic layer 8 each havea magnetization direction perpendicular to the film plane. When a writecurrent is applied to the MTJ element 1A, the magnetization direction ofone of the ferromagnetic layer 2 and the ferromagnetic layer 8 remainsthe same before and after writing, and the magnetization direction ofthe other one is changeable. In this embodiment, the ferromagnetic layer2 is the reference layer, and the ferromagnetic layer 8 is the recordinglayer, for example. A write current flowing in a direction perpendicularto the film plane is applied between the ferromagnetic layer 2 and theferromagnetic layer 8, as in the first embodiment. It should be notedthat the interfacial magnetic layers 3 and 6 are employed to increasespin polarization, as in the first embodiment.

In the second embodiment, the ferromagnetic layer 2, the interfacialmagnetic layer 3, the nonmagnetic layer 4, the interfacial magneticlayer 6, and the ferromagnetic layer 8 are stacked in this order on thebase layer 100. However, those layers can be stacked in reverse order onthe base layer 100. That is, the ferromagnetic layer 8, the interfacialmagnetic layer 6, the nonmagnetic layer 4, the interfacial magneticlayer 3, and the ferromagnetic layer 2 can be stacked in this order onthe base layer 100.

Third Embodiment

FIG. 4 shows a magnetoresistive element according to a third embodiment.FIG. 4 is a cross-sectional view of the magnetoresistive element 1B ofthe third embodiment. The magnetoresistive element 1B of this embodimentis a MTJ element, and has a structure that is formed by stacking aferromagnetic layer 2, an interfacial magnetic layer 3, a nonmagneticlayer 4, and a ferromagnetic layer 10 in this order on a base layer 100.The ferromagnetic layer 2 and the ferromagnetic layer 10 contain Mn andGa, and the composition ratio between the Mn and Ga is Mn_(x)Ga_(100-x)(45≦x<64 atomic %). In this case, the Mn composition rate in theferromagnetic layer 10 is made to differ from the Mn composition rate inthe ferromagnetic layer 2, so as to cause a difference in the coerciveforce between the ferromagnetic layer 10 and the ferromagnetic layer 2.The ferromagnetic layer with the lower Mn composition rate serves as therecording layer, and the ferromagnetic layer with the higher Mncomposition rate serves as the reference layer. A coercive forcedifference of 1 kOe or larger is sufficient between the reference layerand the recording layer. If a ferromagnetic layer with Mn₅₀Ga₅₀ is therecording layer, a ferromagnetic layer of MnGa having a Mn compositionrate of 55 atomic % or higher is suited to be the reference layeraccording to FIG. 2. Also, MnAl is preferably used as the interfacialmagnetic layer 3, so as to increase spin polarization. In the thirdembodiment, the later described material can be used as the nonmagneticlayer 4. As the interfacial magnetic layer 3, MnGa can also be used.

As in the first and second embodiments, the MTJ element 1B of thirdembodiment is also a so-called perpendicular-magnetization MTJ element.Also, as in the first and second embodiments, the interfacial magneticlayer 3 is employed to increase spin polarization.

In the third embodiment, the ferromagnetic layer 2, the interfacialmagnetic layer 3, the nonmagnetic layer 4, and the ferromagnetic layer10 are stacked in this order on the base layer 100. However, thoselayers can be stacked in reverse order on the base layer 100. That is,the ferromagnetic layer 10, the nonmagnetic layer 4, the interfacialmagnetic layer 3, and the ferromagnetic layer 2 can be stacked in thisorder on the base layer 100.

Fourth Embodiment

FIG. 5 shows a magnetoresistive element according to a fourthembodiment. FIG. 5 is a cross-sectional view of the magnetoresistiveelement 1C of the fourth embodiment. The magnetoresistive element 1C ofthis embodiment is a MTJ element, and has a structure that is formed bystacking a ferromagnetic layer 2, an interfacial magnetic layer 3, anonmagnetic layer 4, an interfacial magnetic layer 9, and aferromagnetic layer 10 in this order on a base layer 100. Theferromagnetic layer 2 and the ferromagnetic layer 10 contain Mn and Ga,and the composition ratio between the Mn and Ga is Mn_(x)Ga_(100-x)(45≦x<64 atomic %). In the fourth embodiment, a difference in thecoercive force is caused between the ferromagnetic layer 10 and theferromagnetic layer 2, to differ the Mn composition rates from eachother, as in the third embodiment. Also, MnAl is preferably used as theinterfacial magnetic layers 3 and 9, so as to increase spinpolarization. And also, the interfacial magnetic layer 3 and 9, can bean alloy containing at least one element selected from the groupconsisting of Fe and Co, and at least one element selected from thegroup consisting of Cr, Ni, B, C, P, Ta, Ti, Mo, Al, Si, W, Nb, Mn, andGe. For example, the alloy can be CoFeB, but can also be CoFeSi, CoFeP,CoFeW, CoFeNb, CoFeAl, CoFeAlSi, CoMnSi, CoMnSiAl or the like. In thefourth embodiment, the later described material can be used as thenonmagnetic layer 4. As the interfacial magnetic layers 3 and 9, MnGacan also be used.

As in the first, second, and third embodiments, the MTJ element 1C ofthe fourth embodiment is also a so-called perpendicular-magnetizationMTJ element.

Fifth Embodiment

FIG. 6 shows a magnetoresistive element according to a fifth embodiment.FIG. 6 is a cross-sectional view of the magnetoresistive element 1D ofthe fifth embodiment. The magnetoresistive element 1D of this embodimentis the same as the magnetoresistive element of the fourth embodiment,except that a nonmagnetic layer 12 and a ferromagnetic layer 14 arefurther stacked on the ferromagnetic layer 10. In this embodiment, theinterfacial magnetic layer 9 and the ferromagnetic layer 10 serve as thereference layer. The ferromagnetic layer 14 is also called a bias layer,and has a magnetization direction antiparallel (the opposite from) themagnetization direction of the ferromagnetic layer 10. The ferromagneticlayer 14 can be antiferromagnetically coupled to the ferromagnetic layer10 via the nonmagnetic layer 12 (SAF (Synthetic Anti-Ferromagnetic)coupling). With this arrangement, it is possible to reduce and adjust ashift in the inversion current in the recording layer formed by theinterfacial magnetic layer 3 and the ferromagnetic layer 2, with theshift being caused due to a leak magnetic field from the reference layerformed by the interfacial magnetic layer 9 and the ferromagnetic layer10. The nonmagnetic layer 12 preferably has good thermal stability sothat the ferromagnetic layer 10 and the bias layer 14 do not mix witheach other in a heating process. The nonmagnetic layer 12 alsopreferably has a function to control the crystalline orientation whenthe bias layer 14 is formed.

Further, if the nonmagnetic layer 12 is thick, the distance between thebias layer 14 and the recording layer (the ferromagnetic layer 2 in thisembodiment) is long, and the shift adjusting magnetic field to beapplied from the bias layer 14 to the recording layer is small.Therefore, the film thickness of the nonmagnetic layer 12 is preferably5 nm or smaller. The bias layer 14 is made of a ferromagnetic materialthat has an axis of easy magnetization in a direction perpendicular tothe film plane. Since the bias layer 14 is further away from therecording layer than the reference layer is, the film thickness or thetotal saturation magnetization M_(S)t of the bias layer 14 needs to begreater than that of the reference layer so that the bias layer 14adjusts the leak magnetic field to be applied to the recording layer.That is, according to the results of the study made by the inventors,where t₂ and M_(S2) represent the film thickness and saturationmagnetization of the reference layer, and t₃ and M_(S3) represent thefilm thickness and saturation magnetization of the ferromagnetic layer14 (the bias layer), the following relational expression should besatisfied:M _(S2) ×t ₂ <M _(S3) ×t ₃

The bias layer 14 of the fifth embodiment can also be applied to themagnetoresistive elements of the first through third embodiments. Inthat case, the nonmagnetic layer 12 is stacked on the ferromagneticlayer 8 or the ferromagnetic layer 10 to be the reference layer, and isinterposed between the bias layer 14 and the reference layer.

Next, the respective layers in the MTJ elements 1, 1A, 1B, 1C, and 1Daccording to the first through fifth embodiments are described indetail. Specifically, the ferromagnetic layer 2, the ferromagnetic layer10, the base layer 100, the interfacial magnetic layer 3, theinterfacial magnetic layer 9, the ferromagnetic layer 8, the interfacialmagnetic layer 6, and the nonmagnetic layer 4 are described in thisorder.

(Material of Ferromagnetic Layer 2 and Ferromagnetic Layer 10)

The ferromagnetic layer 2 and the ferromagnetic layer 10 each have anaxis of easy magnetization in a direction perpendicular to the filmplane. The material used as the ferromagnetic layer 2 and theferromagnetic layer 10 is a MnGa alloy containing Mn and Ga, and thecomposition ratio between the Mn and Ga is Mn_(x)Ga_(100-x) (45≦x<64atomic %). By adjusting the Mn composition rate, the saturationmagnetization and the magnetic crystalline anisotropy can be controlled.Also, since a MnGa alloy has a high spin polarization, a highmagnetoresistance ratio can be achieved.

Normally, there is a correlation between the Gilbert damping parameterand the spin orbit interaction of the material. A high-atomic-numbermaterial has a large spin orbit interaction, and has a large Gilbertdamping parameter. Since MnGa is a material formed by light elements,the Gilbert damping parameter is low. Accordingly, only a small energyis required for a magnetization reversal, and the current densityrequired for switching a magnetization direction with spin-polarizedelectrons can be greatly lowered.

(Base Layer 100)

The base layer 100 is used to control the crystalline properties such asthe crystalline orientations and grain sizes of the ferromagnetic layer2 and the layers located above the ferromagnetic layer 2. Therefore, itis essential to select an appropriate material for the base layer 100.In the following, the material and structure of the base layer 100 aredescribed.

A first example of the base layer 100 is a nitride layer that has a(001) oriented NaCl structure, and contains at least one elementselected from the group consisting of Ti, Zr, Nb, V, Hf, Ta, Mo, W, B,Al, and Ce.

A second example of the base layer 100 is a single layer of a (001)oriented perovskite oxide made of ABO₃. Here, the A-site contains atleast one element selected from the group consisting of Sr, Ce, Dy, La,K, Ca, Na, Pb, and Ba, and the B-site contains at least one elementselected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Nb,Mo, Ru, Ir, Ta, Ce, and Pb.

A third example of the base layer 100 is an oxide layer that has a (001)oriented NaCl structure, and contains at least one element selected fromthe group consisting of Mg, Al, and Ce.

A fourth example of the base layer 100 is a layer that has a tetragonalor cubic structure, is (001) oriented, and contains at least one elementselected from the group consisting of Al, Cr, Fe, Co, Rh, Pd, Ag, Ir,Pt, and Au.

A fifth example of the base layer 100 has a stacked structure in which alayer 100 b of the fourth example is stacked on a layer 100 a of one ofthe first through third examples, as shown in FIG. 7, for example. FIG.8 shows an example of the magnetization curves of a structure expressedas “5-nm thick Ta/30-nm thick Mn₇₅Ga₂₅/2.5-nm thick MgO/MgO substrate”.This structure was formed by inserting a 2.5-nm thick MgO layer as abase layer between the MgO substrate and the MnGa layer. According toVSM evaluation, perpendicular magnetic properties including a greatcoercive force of approximately 5 kOe in the perpendicular directionwere achieved.

However, the magnetization loops in the in-plane direction and theperpendicular direction were almost the same. The axis of easymagnetization of Mn₇₅Ga₂₅ is a c-axis (001) direction. Although the filmcrystalline properties were improved, no differences between theperpendicular direction and the in-plane direction were observed whenthe magnetic properties of the entire films were evaluated, supposedlybecause the respective crystal grains had random crystallineorientations and had easy axes of magnetization in random directions.FIG. 9 shows an example of the magnetization curves of a structureexpressed as “5-nm thick Ta/30-nm thick Mn₇₅Ga₂₅/1-nm thick Cr/2.5-nmthick MgO/MgO substrate”. This structure was formed by inserting atwo-layer stacked structure expressed as 1-nm thick Cr/2.5-nm thick MgOlayer as a base layer between the MgO substrate and the MnGa layer.Through VSM evaluation, it was confirmed that the coercive force in theperpendicular direction was approximately 10 kOe, and the coercive forcein the in-plane direction was approximately 3.7 kOe, which is smallerthan that in the direction perpendicular to the film plane. Further, thecoercive force in the perpendicular direction became greater, comparedwith a sample having a single-layer MgO base. By stacking a Cr layer,the MgO surface energy was reduced, and the misfit with the Mn₇₅Ga₂₅ wasreduced. Accordingly, the Mn₇₅Ga₂₅ grew with high crystallineproperties, and the crystalline orientation distribution was restrained.This tendency is also observed with Mn_(x)Ga_(100-x) (45≦x<64 atomic %).By modifying the structure of the base layer in the above manner, theMnGa growth can be controlled, and the magnetic properties can beimproved. Other examples of two-layer stacked structures that can beused as the base layer include Ag/MgO, Ag/TiN, TiN/MgO, Cr/MgO, Cr/TiN,Cr/VN, Cr/NbN, Pt/MgO, Pt/TiN, Pt/VN, Pt/NbN, Ir/MgO, Ir/TiN, Ir/VN, andIr/NbN. Likewise, the same effects as above can be achieved by stackingthree or more layers to serve as the base layer. Examples of base layerseach formed by stacking three or more layers include Cr/Fe/MgO,Cr/Fe/TIN, Ag/Fe/MgO, Cr/TiN/MgO, Ag/Cr/MgO, Cr/VN/MgO, Cr/NbN/MgO,Pt/TiN/MgO, Pt/VN/MgO, Pt/NbN/MgO, Ir/TIN/MgO, Ir/VN/MgO, Ir/NbN/MgO,Cr/Pt/VN/MgO, Cr/Pt/NbN/MgO, and Cr/Pt/TIN/MgO.

(Interfacial Magnetic Layer 3 and Interfacial Magnetic Layer 9)

The interfacial magnetic layer 3 and the interfacial magnetic layer 9each have an axis of easy magnetization in a direction perpendicular tothe film plane. An example of a material that can be used as theinterfacial magnetic layer 3 and the interfacial magnetic layer 9 is analloy containing Mn and Al. A MnAl alloy is a material formed by lightelements, and therefore, has a small Gilbert damping parameter.Accordingly, the energy required for a magnetization reversal is small,and the current density required for switching magnetization directionswith spin-polarized electrons can be greatly lowered. Also, a MnAl alloyhas half-metallic characteristics, since an energy gap exists in one ofthe spin bands of up-spin electrons and down-spin electrons in the (001)direction, and has a high spin polarization. Accordingly, a highmagnetoresistance ratio can be achieved. And also, the interfacialmagnetic layer 3 and 9, can be an alloy containing at least one elementselected from the group consisting of Fe and Co, and at least oneelement selected from the group consisting of Cr, Ni, B, C, P, Ta, Ti,Mo, Al, Si, W, Nb, Mn, and Ge. For example, the alloy can be CoFeB, butcan also be CoFeSi, CoFeP, CoFeW, CoFeNb, CoFeAl, CoFeAlSi, CoMnSi,CoMnSiAl, or the like.

(Ferromagnetic Layer 8)

The ferromagnetic layer 8 has an axis of easy magnetization in adirection perpendicular to the film plane. The material used as theferromagnetic layer 8 can be a metal that has the (111) crystallineorientation of a face-centered cubic (FCC) structure or has the (001)crystalline orientation of a hexagonal close-packed (HCP) structure, ora metal that can form an multilayer, for example. An example of themetal that has the (111) crystalline orientation of a FCC structure orthe (001) crystalline orientation of a HCP structure is an alloycontaining at least one element selected from the first group consistingof Fe, Co, Ni, and Cu, and at least one element selected from the secondgroup consisting of Pt, Pd, Rh, and Au. Specifically, the metal is aferromagnetic alloy such as CoPd, CoPt, NiCo, or NiPt.

The multilayer used in the ferromagnetic layer 8 can be a structure inwhich one element or more of Fe, Co, and Ni or an alloy containing theone element (a ferromagnetic film), and one element of Cr, Pt, Pd, Ir,Rh, Ru, Os, Re, Au, and Cu or an alloy containing the one element (anonmagnetic film) are alternately stacked. For example, the multilayercan be a Co/Pt multilayer, a Co/Pd multilayer, a CoCr/Pt multilayer, aCo/Ru multilayer, a Co/Os multilayer, a Co/Au multilayer, a Ni/Cumultilayer, or the like. In each of the multilayers, the magneticanisotropic energy density and saturation magnetization can becontrolled by adjusting the addition of an element to the ferromagneticfilm and the film thickness ratio between the ferromagnetic film and thenonmagnetic film.

Also, the material used as the ferromagnetic layer 8 can be an alloycontaining at least one element selected from the group consisting oftransition metals Fe, Co, and Ni, and at least one element selected fromthe group consisting of rare-earth metals Tb, Dy, and Gd. For example,the material can be TbFe, TbCo, TbFeCo, DyTbFeCo, or GdTbCo.Alternatively, the material can be a multilayer structure formed byalternately stacking those alloys. Specifically, such a multilayerstructure can be a multilayer film such as a TbFe/Co film, a TbCo/Fefilm, a TbFeCo/CoFe film, a DyFe/Co film, a DyCo/Fe film, or aDyFeCo/CoFe film. In each of those alloys, the magnetic anisotropyenergy density and saturation magnetization can be controlled byadjusting the film thickness ratio and the composition.

The material used as the ferromagnetic layer 8 can also be an alloycontaining at least one element selected from the first group consistingof Fe, Co, Ni, and Cu, and at least one element selected from the secondgroup consisting of Pt, Pd, Rh, and Au. Specifically, the material canbe a ferromagnetic alloy such as FeRh, FePt, FePd, or CoPt.

(Interfacial Magnetic Layer 6)

To increase the magnetoresistance ratio of the magnetoresistive element,a material having a high spin polarization is used as the interfacialmagnetic layer adjacent to the tunnel barrier layer made of MgO. Forexample, the interfacial magnetic layer 6 is preferably made of an alloycontaining at least one metal selected from the group consisting of Feand Co. If an interfacial magnetic layer made of CoFe, a nonmagneticlayer made of MgO, and an interfacial magnetic layer made of CoFe areformed, for example, the epitaxial relationship, CoFe (001)/MgO(001)/CoFe (001), can be formed. In this case, the wavenumberselectivity of tunneling electrons can be improved, and accordingly, ahigh magnetoresistance ratio can be achieved.

It should be noted that, if the interfacial magnetic layer 6 isepitaxially grown in the (001) orientation with respect to MgO, a highmagnetoresistance ratio can be achieved. Therefore, the interfacialmagnetic layer 6 in contact with the nonmagnetic layer 4 made of MgO canexpand and contract in the direction perpendicular to the film plane.

Also, to control the saturation magnetization, at least one elementselected from the group consisting of Cr, Ni, B, C, P, Ta, Ti, Mo, Al,Si, W, Nb, Mn, and Ge can be added to the interfacial magnetic layer 6.That is, the interfacial magnetic layer 6 can be an alloy containing atleast one element selected from the group consisting of Fe and Co, andat least one element selected from the group consisting of Cr, Ni, B, C,P, Ta, Ti, Mo, Al, Si, W, Nb, Mn, and Ge. For example, the alloy can beCoFeB, but can also be CoFeSi, CoFeP, CoFeW, CoFeNb, CoFeAl, CoFeAlSi,CoMnSi, CoMnSiAl, or the like. Those alloys have the similar spinpolarization as that of CoFeB. Alternatively, the interfacial magneticlayer 6 can be a Heusler metal such as Co₂FeSi, Co₂MnSi, or Co₂MnGe. AHeusler metal has a spin polarization equal to or higher than that ofCoFeB, and therefore, is suited to be an interfacial magnetic layer.

(Nonmagnetic Layer 4)

The nonmagnetic layer 4 is made of an insulating material, andtherefore, a tunnel barrier layer is used as the nonmagnetic layer 4.The tunnel barrier layer material can be an oxide having a maincomponent that is at least one element selected from the groupconsisting of magnesium (Mg), calcium (Ca), barium (Ba), aluminum (Al),silver (Ag), copper (Cu), beryllium (Be), strontium (Sr), zinc (Zn), andtitanium (Ti). Specifically, the tunnel barrier layer material can beMgO, AlO, ZnO, SrO, TiO, or the like. Alternatively, the tunnel barrierlayer can be a mixed crystalline material containing two or morematerials selected from the above mentioned oxide group, or can be astacked structure of those materials. Examples of mixed crystallinematerials include MgAlO, MgZnO, MgTiO, and MgCaO. Examples of two-layerstacked structures include MgO/ZnO, MgO/AlO, TiO/AlO, and MgAlO/MgO.Examples of three-layer stacked structures include AlO/MgO/AlO,MgO/ZnO/MgO, and ZnO/MgO/ZnO.

The tunnel barrier layer can be either a crystalline material or anamorphous material. If the tunnel barrier layer is a crystalline layer,however, electron scattering can be restrained in the tunnel barrier,and the probability of selective tunneling conduction of electrons froma ferromagnetic layer while wavenumber is kept becomes higher.Accordingly, the magnetoresistance ratio can be made higher. Therefore,to achieve a high magnetoresistance ratio, it is preferable to use acrystalline tunnel barrier.

For example, in a case where a stacked structure is formed by stacking aferromagnetic layer made of MnGa, a nonmagnetic layer made ofcrystalline MgO, and a ferromagnetic layer made of MnGa in this order,the epitaxial relationship, MnGa (001)/MgO (001)/MnGa (001), can beformed. Accordingly, the wavenumber selectivity of tunneling electronscan be improved, and a high magnetoresistance ratio can be achieved.However, the lattice mismatch determined from the lattice constants ofbulks in the film in-plane directions of MnGa and MgO is as large asapproximately 7.7%. The lattice mismatch is defined by the followingmathematical formula: (a(MgO)−a(MnGa))/a(MnGa)×100. Here, a(MgO) anda(MnGa) represent the lattice constants of MnGa and MgO in the filmin-plane directions, respectively. If the lattice mismatch is large, adislocation or the like is caused in an interface so as to reduce theinterfacial energy generated due to lattice distortion. In that case, anepitaxial relationship is formed between crystal grains, and it isdifficult to cause uniform epitaxial growth in the film plane. When acurrent is applied to the element, a dislocation becomes the scatteringsource of electrons, and the magnetoresistance ratio becomes lower.Therefore, to cause uniform epitaxial growth in the film plane without adislocation, it is essential to form a stacked structure with materialshaving a small lattice mismatch. Therefore, MgAlO is used as thenonmagnetic layer 4, and MnGa (001)/MgAlO (001)/MnGa (001) is formed.Accordingly, the lattice mismatch can be reduced to 3.5%, and a highermagnetoresistance ratio is achieved.

EXAMPLES

Next, the stacked structures of specific perpendicular-magnetization MTJelements are described as examples. The magnetoresistive films (samples)described below were formed to manufacture perpendicular-magnetizationMTJ elements of one of the first through fifth embodiments. After thefilm formation, vacuum annealing is performed at an appropriatetemperature for an appropriate period of time, so as to optimize TMRcharacteristics and magnetic characteristics.

Example 1

As a perpendicular-magnetization MTJ element of Example 1, the MTJelement 1 having perpendicular magnetization according to the firstembodiment illustrated in FIG. 1 is formed. The MTJ element 1 accordingto Example 1 has a structure formed by stacking a base layer 100, aferromagnetic layer 2, a nonmagnetic layer 4, an interfacial magneticlayer 6, and a ferromagnetic layer 8 in this order on a MgOsingle-crystal substrate (not shown). Here, a 20-nm thick MnGa film isused as the ferromagnetic layer 2. The ferromagnetic layer 2 has an axisof easy magnetization in a direction perpendicular to the film plane,and serves as the recording layer having a changeable magnetizationdirection. A 16-nm thick TbCoFe film is used as the ferromagnetic layer8. The ferromagnetic layer 8 has an axis of easy magnetization in adirection perpendicular to the film plane, and serves as the referencelayer having a fixed magnetization direction. A 1-nm thick Co₆₀Fe₂₀B₂₀film is used as the interfacial magnetic layer 6. A tunnel barrier layermade of MgAlO is used as the nonmagnetic layer 4.

The electrical resistance values were measured at room temperature,while a magnetic field was swept in a direction perpendicular to thefilm plane of the perpendicular MTJ. The results show that the ratio ofchange in electrical resistance between a case where the magnetizationdirections of the recording layer and the reference layer are paralleland a case where the magnetization directions are antiparallel is 27%.

Example 2

As a perpendicular-magnetization MTJ element of Example 2, theperpendicular-magnetization MTJ element 1A according to the secondembodiment illustrated in FIG. 3 was formed. The MTJ element 1Aaccording to Example 2 has a structure formed by stacking a base layer100, a ferromagnetic layer 2, an interfacial magnetic layer 3, anonmagnetic layer 4, an interfacial magnetic layer 6, and aferromagnetic layer 8 in this order on a MgO single-crystal substrate(not shown).

Here, a 20-nm thick MnGa film is used as the ferromagnetic layer 2. A20-nm thick MnAl film is used as the interfacial magnetic layer 3. A1-nm thick Co₆₀Fe₂₀B₂₀ film is used as the interfacial magnetic layer 6.A 16-nm thick TbCoFe film is used as the ferromagnetic layer 8. A tunnelbarrier layer made of MgAlO is used as the nonmagnetic layer 4.

The electrical resistance values were measured at room temperature,while a magnetic field was swept in a direction perpendicular to thefilm plane of the perpendicular MTJ. The results show that the ratio ofchange in electrical resistance between a case where the magnetizationdirections of the recording layer and the reference layer are paralleland a case where the magnetization directions are antiparallel is 43%.

Example 3

As a perpendicular-magnetization MTJ element of Example 3, theperpendicular-magnetization MTJ element 1B according to the thirdembodiment illustrated in FIG. 4 is formed. The MTJ element 1B accordingto Example 3 has a structure formed by stacking a base layer 100, aferromagnetic layer 2, an interfacial magnetic layer 3, a nonmagneticlayer (a tunnel barrier layer) 4, and a ferromagnetic layer 10 in thisorder on a MgO single-crystal substrate (not shown). Here, a 20-nm thickMn₅₀Ga₅₀ film is used as the ferromagnetic layer 2. A 20-nm thick MnAlfilm is used as the interfacial magnetic layer 3. A 50-nm thick Mn₆₄Ga₃₆film is used as the ferromagnetic layer 10. A tunnel barrier layer madeof MgO is used as the nonmagnetic layer 4.

The electrical resistance values were measured at room temperature,while a magnetic field was swept in a direction perpendicular to thefilm plane of the perpendicular MTJ. The results show that the ratio ofchange in electrical resistance between a case where the magnetizationdirections of the recording layer and the reference layer are paralleland a case where the magnetization directions are antiparallel is 23%.

Example 4

As a perpendicular-magnetization MTJ element of Example 4, theperpendicular-magnetization MTJ element 1C according to the fourthembodiment illustrated in FIG. 5 is formed. The MTJ element 1C accordingto Example 4 has a structure formed by stacking a base layer 100, aferromagnetic layer 2, an interfacial magnetic layer 3, a nonmagneticlayer 4, an interfacial magnetic layer 9, and a ferromagnetic layer 10in this order on a MgO single-crystal substrate (not shown).

Here, a 20-nm thick Mn₅₀Ga₅₀ film is used as the ferromagnetic layer 2.A 20-nm thick MnAl film is used as the interfacial magnetic layer 3. A20-nm thick MnAl film is used as the interfacial magnetic layer 9. A50-nm thick Mn₆₄Ga₃₆ film is used as the ferromagnetic layer 10. Atunnel barrier layer made of MgO is used as the nonmagnetic layer 4.

The electrical resistance values were measured at room temperature,while a magnetic field was swept in a direction perpendicular to thefilm plane of the perpendicular MTJ. The results show that the ratio ofchange in electrical resistance between a case where the magnetizationdirections of the recording layer and the reference layer are paralleland a case where the magnetization directions are antiparallel is 19%.

As described so far, each of the embodiments and Examples can provide amagnetoresistive element that has perpendicular magnetic anisotropy andis capable of achieving a greater magnetoresistive effect.

Sixth Embodiment

The MTJ elements 1, 1A, 1B, 1C, and 1D of the first through fifthembodiments can be applied to MRAMs. In the following, for ease ofexplanation, an example case where the MTJ element 1 of the firstembodiment is used is described.

Each memory element forming a MRAM includes a recording layer that has achangeable (or reversible) magnetization (or spin) direction, areference layer that has an invariable (or fixed) magnetizationdirection, and a nonmagnetic layer interposed between the recordinglayer and the reference layer. Where “the magnetization direction of thereference layer is invariable”, the magnetization direction of thereference layer does not change when the magnetization switching currentto be used for switching the magnetization direction of the recordinglayer is applied to the reference layer. As one of the two ferromagneticlayers each having an axis of easy magnetization in a directionperpendicular to the film plane serves as the recording layer while theother one serves as the reference layer, a MRAM including MTJ elementsas memory elements can be formed.

Specifically, the two ferromagnetic layers are made to have a differencein coercive force from each other, so that the two ferromagnetic layerscan be used as the recording layer and the reference layer. Therefore,in a MTJ element, a ferromagnetic layer having a large inversion currentis used as one ferromagnetic layer (the reference layer), and aferromagnetic layer having a smaller inversion current than theferromagnetic layer serving as the reference layer is used as the otherferromagnetic layer (the recording layer). In this manner, a MTJ elementincluding a ferromagnetic layer with a variable magnetization directionand a ferromagnetic layer with a fixed magnetization direction can berealized.

FIG. 10 is a circuit diagram showing the structure of the MRAM accordingto a sixth embodiment. The MRAM of this embodiment includes memory cellsarranged in a matrix fashion, and each of the memory cells includes theMTJ element 1. One end of each of the MTJ elements 1 is electricallyconnected to a bit line BL. One end of each bit line BL is electricallyconnected to a sense amplifier SA via an N-channel MOS transistor ST1serving as a select switch. The sense amplifier SA compares a readpotential Vr and a reference potential Vref supplied from a MTJ element1, and outputs the result of the comparison as an output signal DATA. Aresistor Rf electrically connected to the sense amplifier SA is afeedback resistor.

The other end of each bit line BL is electrically connected to the drainof a P-channel MOS transistor P1 and the drain of an N-channel MOStransistor N1 via an N-channel MOS transistor ST2 serving as a selectswitch. The source of the MOS transistor P1 is connected to a supplyterminal Vdd, and the source of the MOS transistor N1 is connected to aground terminal Vss.

The other end of each of the MTJ elements 1 is electrically connected toa lower electrode 29. Each of the lower electrodes 29 is electricallyconnected to a source line SL via an N-channel MOS transistor ST3serving as a select switch. It should be noted that the source line SLextends in a direction parallel to the bit lines BL.

The source line SL is electrically connected to the drain of a P-channelMOS transistor P2 and the drain of an N-channel MOS transistor N2 via anN-channel MOS transistor ST4 serving as a select switch. The source ofthe MOS transistor P2 is connected to the supply terminal Vdd, and thesource of the MOS transistor N2 is connected to the ground terminal Vss.The source line SL is also connected to the ground terminal Vss via anN-channel MOS transistor ST5 serving as a select switch.

The gate of each MOS transistor ST3 is electrically connected to a wordline WL. Each word line WL extends in a direction perpendicular to thedirection in which the bit lines BL extend.

Data writing into the MTJ elements 1 is performed by a spin-injectionwriting technique. That is, the direction of the write current flowingin the MTJ elements 1 is controlled by switching on and off the MOStransistors P1, P2, N1, and N2 with control signals A, B, C, and D, soas to realize data writing.

Data reading from the MTJ elements 1 is performed by supplying a readcurrent to the MTJ elements 1. The read current is set at a smallervalue than the write current. Each MTJ element 1 has a resistance valuethat varies depending on whether the magnetization directions of thereference layer and the recording layer are parallel or antiparallel,because of a magnetoresistive effect. That is, the resistance value of aMTJ element 1 becomes the smallest when the magnetization directions ofthe reference layer and the recording layer are parallel, and theresistance value of the MTJ element 1 becomes the largest when themagnetization directions of the reference layer and the recording layerare antiparallel. Such changes in resistance value are detected by thesense amplifier SA, to read the information recorded in the MTJ elements1.

FIG. 11 is a cross-sectional view of one of the above described memorycells. A device isolation insulating layer 22 having a STI (ShallowTrench Isolation) structure is formed in a P-type semiconductorsubstrate 21. The N-channel MOS transistor ST3 as a select switch isformed in the device region (the active region) surrounded by the deviceisolation insulating layer 22. The MOS transistor ST3 includes diffusionregions 23 and 24 serving as source/drain regions, a gate insulatingfilm 25 formed on the channel region between the diffusion regions 23and 24, and a gate electrode 26 formed on the gate insulating film 25.The gate electrode 26 is equivalent to the word lines WL shown in FIG.10.

A contact plug 27 is formed on the diffusion region 23. The source lineSL is formed on the contact plug 27. A contact plug 28 is formed on thediffusion region 24. The lower electrode 29 is formed on the contactplug 28. The MTJ element 1 is provided on the lower electrode 29. Anupper electrode 30 is formed on the MTJ element 1. The bit line BL isprovided on the upper electrode 30. The space between the semiconductorsubstrate 21 and the bit line BL is filled with an interlayer insulatinglayer 31.

An example case where magnetoresistive elements according to one of thefirst through fifth embodiments are used in a MRAM has been described sofar. However, the magnetoresistive elements according to the firstthrough fifth embodiments can also be used in any other devicesutilizing the TMR effect.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel methods and systems describedherein can be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the methods andsystems described herein can be made without departing from the spiritof the inventions. The accompanying claims and their equivalents areintended to cover such forms or modifications as would fall within thescope and spirit of the inventions.

What is claimed is:
 1. A magnetoresistive element comprising: a baselayer; a first magnetic layer provided on the base layer, and includinga first magnetic film having an axis of easy magnetization in adirection perpendicular to a film plane, the first magnetic filmincluding Mn_(x)Ga_(100-x) (45≦x<64 atomic %); a first nonmagnetic layerprovided on the first magnetic layer; a second magnetic layer providedon the first nonmagnetic layer, and including a second magnetic filmhaving an axis of easy magnetization in a direction perpendicular to afilm plane, the second magnetic film including Mn_(y)Ga_(100-y) (45≦y<64atomic %); and an interfacial layer provided between the first magneticlayer and the first nonmagnetic layer, and/or between the secondmagnetic layer and the first nonmagnetic layer, the interfacial layerincluding a Heusler alloy, the first and second magnetic layerscomprising different Mn composition rates from each other, the firstmagnetic layer having a smaller Mn concentration than the secondmagnetic layer, a magnetization direction of the first magnetic layerbeing changeable and a magnetization direction of the second magneticlayer being unchangeable.
 2. The magnetoresistive element according toclaim 1, wherein the interfacial layer includes one of Co₂FeSi, Co₂MnSi,and Co₂MnGe.
 3. The magnetoresistive element according to claim 1,wherein the first nonmagnetic layer is an oxide that contains oneelement selected from the group consisting of Mg, Ca, Al, Sr, and Ti. 4.The magnetoresistive element according to claim 1, wherein the baselayer includes a nitride layer that has a (001) oriented NaCl structure,the nitride layer containing at least one element selected from thegroup consisting of Ti, Zr, Nb, V, Hf, Ta, Mo, W, B, Al, and Ce.
 5. Themagnetoresistive element according to claim 1, wherein the base layerincludes a (001) oriented perovskite oxide that is made of ABO₃, theA-site containing at least one element selected from the groupconsisting of Sr, Ce, Dy, La, K, Ca, Na, Pb, and Ba, the B-sitecontaining at least one element selected from the group consisting ofTi, V, Cr, Mn, Fe, Co, Ni, Ga, Nb, Mo, Ru, Ir, Ta, Ce, and Pb.
 6. Themagnetoresistive element according to claim 1, wherein the base layerincludes an oxide layer that has a (001) oriented NaCl structure, theoxide layer containing at least one element selected from the groupconsisting of Mg, Al, and Ce.
 7. The magnetoresistive element accordingto claim 1, wherein the base layer comprises one of a tetragonalstructure and a cubic structure, is (001) oriented, and contains atleast one element selected from the group consisting of Al, Cr, Fe, Co,Rh, Pd, Ag, Ir, Pt, and Au.
 8. The magnetoresistive element according toclaim 1, wherein the base layer includes a first layer and a secondlayer provided on the first layer, the first layer includes at least oneof: a nitride layer that has a (001) oriented NaCl structure, andcontains at least one element selected from the group consisting of Ti,Zr, Nb, V, Hf, Ta, Mo, W, B, Al, and Ce; a (001) oriented perovskiteoxide layer that is made of ABO₃, the A-site containing at least oneelement selected from the group consisting of Sr, Ce, Dy, La, K, Ca, Na,Pb, and Ba, the B-site containing at least one element selected from thegroup consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Nb, Mo, Ru, Ir, Ta,Ce, and Pb; and an oxide layer that has a (001) oriented NaCl structure,and contains at least one element selected from the group consisting ofMg, Al, and Ce, and the second layer has one of a tetragonal structureand a cubic structure, is (001) oriented, and contains at least oneelement selected from the group consisting of Al, Cr, Fe, Co, Rh, Pd,Ag, Ir, Pt, and Au.
 9. The magnetoresistive element according to claim1, wherein the second magnetic layer has a fixed magnetizationdirection, the magnetoresistive element further comprises: a thirdmagnetic layer provided on the opposite side of the second magneticlayer from the first nonmagnetic layer, having an axis of easymagnetization in a direction perpendicular to a film plane, and having amagnetization direction antiparallel to the magnetization direction ofthe second magnetic layer; and a second nonmagnetic layer providedbetween the second ferromagnetic layer and the third ferromagneticlayer, wherein M_(S2) represents a saturation magnetization of thesecond magnetic layer, t₂ represents a film thickness of the secondmagnetic layer, M_(S3) represents a saturation magnetization of thethird magnetic layer, and t₃ represents a film thickness of the thirdmagnetic layer, and the following relationship is satisfiedM _(S2) ×t ₂ <M _(S3) ×t ₃.
 10. A magnetic memory comprising: themagnetoresistive element according to claim 1, a first interconnect thatis electrically connected to the first magnetic layer of themagnetoresistive element; and a second interconnect that is electricallyconnected to the second magnetic layer of the magnetoresistive element.11. A magnetoresistive element comprising: a base layer; and a stackedstructure provided on the base layer, the stacked structure including: afirst magnetic layer including a first magnetic film having an axis ofeasy magnetization in a direction perpendicular to a film plane, thefirst magnetic film including Mn_(x)Ga_(100-x) (45≦x<64 atomic %); asecond magnetic layer including a second magnetic film having an axis ofeasy magnetization in a direction perpendicular to a film plane; a firstnonmagnetic layer provided between the first magnetic layer and thesecond magnetic layer, and containing at least one element selected fromthe group consisting of Mg, Ca, Ba, Al, Ag, Cu, Be, Sr, Zn, and Ti; anda first interfacial layer provided between the first magnetic layer andthe first nonmagnetic layer, the first interfacial layer including aHeusler alloy, a magnetization direction of the first magnetic layerbeing changeable, wherein the second magnetic layer has a fixedmagnetization direction, wherein the magnetoresistive element furthercomprises: a third magnetic layer provided on the opposite side of thesecond magnetic layer from the first nonmagnetic layer, having an axisof easy magnetization in a direction perpendicular to a film plane, andhaving a magnetization direction antiparallel to the magnetizationdirection of the second magnetic layer; and a second nonmagnetic layerprovided between the second ferromagnetic layer and the thirdferromagnetic layer, wherein M_(S2) represents a saturationmagnetization of the second magnetic layer, t₂ represents a filmthickness of the second magnetic layer, M_(S3) represents a saturationmagnetization of the third magnetic layer, and t₃ represents a filmthickness of the third magnetic layer, and the following relationship issatisfiedM _(S2) ×t ₂ <M _(S3) ×t ₃.
 12. The magnetoresistive element accordingto claim 11, wherein the first interfacial layer includes one ofCo₂FeSi, Co₂MnSi, and Co₂MnGe.
 13. The magnetoresistive elementaccording to claim 11, further comprising a second interfacial layerprovided between the second magnetic layer and the first nonmagneticlayer, the second interfacial layer including a Heusler alloy.
 14. Themagnetoresistive element according to claim 11, wherein the base layerincludes a nitride layer that has a (001) oriented NaCl structure, thenitride layer containing at least one element selected from the groupconsisting of Ti, Zr, Nb, V, Hf, Ta, Mo, W, B, Al, and Ce.
 15. Themagnetoresistive element according to claim 11, wherein the base layerincludes a (001) oriented perovskite oxide that is made of ABO₃, theA-site containing at least one element selected from the groupconsisting of Sr, Ce, Dy, La, K, Ca, Na, Pb, and Ba, the B-sitecontaining at least one element selected from the group consisting ofTi, V, Cr, Mn, Fe, Co, Ni, Ga, Nb, Mo, Ru, Ir, Ta, Ce, and Pb.
 16. Themagnetoresistive element according to claim 11, wherein the base layerincludes an oxide layer that has a (001) oriented NaCl structure, theoxide layer containing at least one element selected from the groupconsisting of Mg, Al, and Ce.
 17. The magnetoresistive element accordingto claim 11, wherein the base layer comprises one of a tetragonalstructure and a cubic structure, is (001) oriented, and contains atleast one element selected from the group consisting of Al, Cr, Fe, Co,Rh, Pd, Ag, Ir, Pt, and Au.
 18. The magnetoresistive element accordingto claim 11, wherein the base layer includes a first layer and a secondlayer provided on the first layer, the first layer includes at least oneof: a nitride layer that has a (001) oriented NaCl structure, andcontains at least one element selected from the group consisting of Ti,Zr, Nb, V, Hf, Ta, Mo, W, B, Al, and Ce; a (001) oriented perovskiteoxide layer that is made of ABO₃, the A-site containing at least oneelement selected from the group consisting of Sr, Ce, Dy, La, K, Ca, Na,Pb, and Ba, the B-site containing at least one element selected from thegroup consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Nb, Mo, Ru, Ir, Ta,Ce, and Pb; and an oxide layer that has a (001) oriented NaCl structure,and contains at least one element selected from the group consisting ofMg, Al, and Ce, and the second layer has one of a tetragonal structureand a cubic structure, is (001) oriented, and contains at least oneelement selected from the group consisting of Al, Cr, Fe, Co, Rh, Pd,Ag, Ir, Pt, and Au.
 19. A magnetic memory comprising: themagnetoresistive element according to claim 11; a first interconnectthat is electrically connected to the first magnetic layer of themagnetoresistive element; and a second interconnect that is electricallyconnected to the second magnetic layer of the magnetoresistive element.